Chapter 3. Experimental Details
3.6 Temperature Programmed Apparatus
The schematic representation of the apparatus utilised in this work is shown in Figure 3.1. The system was based on that used by Baker and Metcalfe, [4] and consisted of two separate frames. All equipment was designed and built in house. The first frame housed five gas lines. The first of these was attached to oxygen (SGE 1/8”oxygen trap) and water (1/8” Safe Glass Moisture Trap, CRS systems) filters. All the filters had 1/8” brass on/off valves (Swagelok) situated at either end in order to protect the filters during any subsequent adjustment of the systems’ configuration. After the filters, the line passed to a mass flow controller (UNIT 7300, calibrated for N2,maximum flow rate of 250 ml min-1).
This first gas line was primarily used as the carrier gas line (Ar). Three further gas lines passed through a similar series of valves and filters and into a four way selection valve (Swagelok, 5-way switching ball valve), which allowed the relevant flow to be selected. Typically, these gas lines were used for dilute hydrogen (5% H2/Ar), methane (5%
CH4/Ar) and oxygen (5% O2/Ar). The O2/Ar line had no oxygen trap. A calibration gas
mixture was also connected to one of the inlet ports of the four way selection valve. This mixture was formulated so that the mass spectrometer could be calibrated for several species. A mass flow controller (UNIT 7300, calibrated for N2 max flow rate600ml min-1)
was placed in-line after the selection valve in order to regulate the flow of gas. All tubing utilised on the first frame was 1/8” stainless steel tubing (SGE). The carrier gas and the selected treatment or calibration gas both passed into a two-way switching valve (Rheodyne 3000-038) positioned on the second frame. The design of this valve meant that flows from the two mass flow controllers would be continuous with one flow directed to vent and one to the rest of the apparatus. In this way the samples could be pre-treated with one gas and then the flow switched to an alternate gas with no delay or risk of air entering the equipment. The gas going to vent passed through a rotameter (Cole-Palmer, 50ml) and a bubble flow meter so that the flow rate could be measured if required. The other gas flow passed through a liquid injection port. This consisted of a 1/16” stainless steel T-junction, with two of the arms of the T being assigned to the gas entering and exiting the junction and the third holding a 1/16” tubing to 1/4” (Swagelok) reducing fitting. The 1/4” fitting
contained two septa which created a gas tight seal. This arrangement allowed pulses of water of known volume to be injected into the system through the septa, using a 1.0µl gas- tight syringe (SGE, 1BR-7) in order to calibrate the mass spectrometer. Downstream of the liquid injection port all tubing was heated using a heating cord which was wound around the tubing to prevent water or other species from condensing in the apparatus.
Next in line was a 6 way switching valve (Rheodyne, 7 port stainless steel switching valve), downstream from which all tubing was 1/16” glass-lined tubing (SGE, 0.8mm I.D.) the reason for this was two-fold. First, it reduced the volume of the system and hence the time lag between desorption of a species from the sample and its detection in the mass spectrometer. Second, it had the effect of minimising any adsorption of products onto the inner walls of the system. The Rheodyne switching valve could direct the gas flow to either one of the two reactors or the by-pass line. An identical valve after the reactors was used to select the desired flow path. The by-pass line was simply an uninterrupted, heated piece of glass-lined tubing through which a flow could be diverted whilst the micro-reactors were being attached or removed.
The reactor arrangement is shown in Figure 3.2. The quartz micro-reactors were built in- house and consisted of 1/4” o.d. tubes which broadened out to 1/2” o.d. in the centre. In this central bubble there was a quartz frit on which the sample was located (Figure 3.3). Typical sample mass was 50 mg. This ensured that the quartz frit was completely covered in sample without there being excessive depth of catalyst bed which may have resulted in re-adsorption of desorbed species in the downstream part of the catalyst bed. The nature of the micro-reactor employed meant that only powder samples were suitable for investigation as they were required to fit through the thin neck of the micro-reactor. The micro-reactor was attached using 1/4” to 1/16” reducing union compression fittings (SGE) at the bottom and a three way reducing union which consisted of 2 × 1/16” ports and one 1/4” port (SGE). Each of these employed graphite ferrules due to the high temperatures to which they would be subjected.
MFC
MFC
Vent Vent Gas injection Reactor 1 Reactor 2 By-pass Line O2Trap H2O Traps To Mass Spec Needle Valve Selection valve On/Off Valve Calibration Gas Liquid injection To Mass Spec Ar H2/Ar CH4/Ar O2/Ar = Valve = O2Filter = Rotameter= liquid injection valve
= Gas line
= Gas injection valve = H2O Filter
= Bubble flow meter = Heated Gas line
Key
Figure 3.1 Apparatus for temperature programmed experiments.
The furnaces were both designed and built in-house, they consisted of a 300 mm by 25 mm i.d. quartz tube encased in a 115 mm diameter steel shell containing insulation. Two K- type thermocouples were employed. One was located in the centre of the furnace tube and regulated the power input to the furnace and a second thermocouple was held in position against the outside of the reactor next to the quartz frit on which the sample was located. The reading from the second thermocouple was logged by the computer attached to the
mass spectrometer. During a heating programme the top and bottom of the micro-reactor were surrounded with kaowool insulation which served the dual purpose of ensuring a better heat distribution within the furnace as well as enabling the micro-reactor to be secured centrally within the quartz tube. After passing through the reactor and selection valve the gas passed through a 1/16” needle valve which enabled a small proportion of the gas to be fed to the mass spectrometer via a heated 1/32” capillary, with the remainder going to vent through the rotameter and bubble flow meter mentioned above.
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Tube furnace
Thermocouple Quartz micro-reactor
Quartz frit Metal ring
Gas flow
Figure 3.2 Furnace and reactor arrangement.
There were five gases or gas mixtures employed during this investigation. These were provided from gas cylinders (BOC). These cylinders were connected via pressure regulators and Swagelok on/off valves to the experimental rig. All lines were 1/8” PTFE tubing with PTFE ferrules. Pure Ar (BOC, standard grade) was employed as a carrier gas for desorption experiments and also during water calibration measurements with oxygen
1/2” o.d. quartz bulb
and H2O filters employed as previously stated. The gas mixtures employed were 5% H2/Ar,
5% CH4/Ar and 5% O2/Ar (all BOC, standard grade) and a calibration gas mixture
containing 0.1023% ethane, 0.1032% ethene, 1.02% CO2, 2.01% H2, 2.03% CO, 0.46%
O2 and a balance of Ar (BOC, certification level +/- 5% analytical accuracy). The typical
flow rate employed during experiments was 50 ml min-1. This was regulated by the mass flow controllers which were set using a calibration factor table obtained from Brookes instruments for each gas mixture employed. To check the accuracy of the calibration factors the flow rate was also measured using a bubble flow meter and flow rates were recorded.
Figure 3.3 Quartz micro-reactor.